1. Field of the Invention
The present invention relates to a laser processing apparatus and a laser processing method. The present invention relates more particularly, although not exclusively, to a laser processing apparatus and a laser processing method which are adapted to concentrate laser light into a material to be cut.
2. Description of the Related Art
There are various techniques for separating a surface of a material, which is to be cut, into a plurality of areas. For example, a blade dicing method adapted to rotate a circular blade, which has a width ranging from tens μm to hundreds μm, at a high speed to cut a semiconductor substrate, (e.g., a silicon wafer), into semiconductor chips is known. Note that the substrate is abraded by abrasives provided on the surface of the blade that cuts the substrate. Coolant water is sprayed onto a cutting surface to reduce heat generation and abrasion associated with the cutting of the substrate. However, dusts, (e.g., chips produced by cutting the substrate itself), fine particles of abrasives, and particles of adhesive of an adhesive tape used to fix the substrate and a processing table, get mixed into the coolant water and are widely scattered. Especially, in a case where the substrate is a semiconductor substrate, many minute function elements are formed on a surface of the substrate. Thus, there is a fear that dust can seriously contaminate and affect the reliability of the function elements.
To facilitate solving this problem, the cutting of a substrate can be performed in a dry environment without using coolant water. Thus, a processing method of cutting the substrate by concentrating laser light, having a wavelength, at which the laser light is highly absorbed by the substrate, onto a surface of the substrate can be used. However, this method has drawbacks where parts of the surface of the substrate, which surround a cut part, are thermally melted. The thermal damage can result in damaged logic circuits generated from the substrate. Additionally problems due to dusts still occur, from dust produced by resolidification of melted materials which adhere to the surface of the substrate.
Japanese Patent Application Laid-Open Nos. 2002-192370 and 2002-205180 discuss examples of a processing method of cutting a substrate by concentrating highly absorbable laser light into a substrate. These methods employ an internal processing region formed by concentrating laser light having a specific wavelength, at which the laser light has high permeability for the substrate, into the substrate as a starting point for cutting the substrate. Thus, no melted regions are formed on the surface of the substrate. Consequently, these methods enable dust-reduced cutting.
The laser light used to form such a modified layer is concentrated into the substrate through a microscope objective lens used for processing-position observation. Laser processing is performed by simultaneously causing an actuator, (e.g., a stage actuator), to drive the substrate along a predetermined cutting line while maintaining the concentrated condition of the laser light.
However, according to the aforementioned method, to accurately cut the substrate, which is an example of the material to be cut, along a predetermined cutting line on the surface of the substrate, laser light can be concentrated into that part of the substrate which is located just under the predetermined cutting line. To that end, one can keep observing a state in which laser light is irradiated onto the surface of the substrate, by using the microscope objective lens even during the laser processing. However, because the microscope objective lens is also used to concentrate laser light to a concentrating position inside the substrate, the microscope objective lens can simultaneously perform both the functions of observing the surface of the substrate and concentrating laser light onto a given position inside the substrate. In this regard, Japanese Patent Application Laid-Open No. 2004-188422 discusses an apparatus adapted to concentrate both of laser light for focusing and laser light for processing to the surface of an object to be processed, by using the same condenser lens, which faces the surface of the object. However, in the apparatus discussed therein, when the condenser lens is moved during focusing, the focal position of laser light for processing can be displaced.
To maintain an irradiating position of laser light at a given position inside a substrate, it is required that a processing system processes the surface of the substrate, which takes into consideration the deformation of a pedestal under the substrate and the distortion of the substrate. In this case, usually, a predetermined laser processing is performed by using an automatic focusing mechanism to focus the microscope objective lens on the surface of the substrate. However, since the microscope objective lens is incorporated into the automatic focusing mechanism in this case, a laser light concentrating optical system cannot concentrate laser light to a given position inside the substrate during a period when the focal position of an observing optical system is being adjusted.
Especially, in a case where the substrate is a silicon wafer, there can sometimes be a misalignment between the predetermined cutting line and a laser light concentrating point that can be located just under the predetermined cutting line, due to an industrial error caused when the substrate and devices are formed. In this case, according to the aforementioned laser processing method, it is highly likely that a crack will deviate from the predetermined cutting line with the crack developing towards the surface of the substrate, which can result in damaged logic circuits. Furthermore such disparity between the desired cutting position and the actual crack formation can result in other structures being damaged. For example, in a device substrate of a liquid discharge head in which a discharge port, (e.g., an inkjet nozzle), is formed, an opening structure is adapted to supply liquid, (e.g., ink), under the discharge port. Disparities in the cutting can result in a crack that extends through such a structure, so that the device substrate becomes damaged.